Ductile iron (also called nodular cast iron) is a cast iron that contains carbon in the form of graphite spheroids/nodules. Due to their shape, these small spheroids/nodules of graphite are better at reducing stress than the finely dispersed graphite flakes in grey iron, thereby imparting greater tensile strength thereto as compared with other types of iron.
Austempered ductile iron (ADI) (which is sometimes referred to as “bainitic ductile iron” even though when correctly heat treated, ADI contains little or no bainite due to the high silicon level compared to common steels) represents a special family of ductile iron alloys which possess improved strength and ductility properties as a result of a heat treatment called “austempering”.
In a typical austempering heat treatment cycle a casting is firstly heated and then held at an austenitizing temperature until the casting becomes fully austenitic and the matrix becomes saturated with carbon. After the casting is fully austenitized, it is quenched in a salt bath at a quenching rate that is high enough to avoid the formation of pearlite (or solution strengthened ferrite) during the quenching. The casting is then held at temperature called the “austempering” temperature. The final microstructure and properties of the ADI casting are usually considered to be determined mainly by the austempering temperature and the holding time. After said isothermal austempering, the casting is cooled to room temperature.
To accelerate the solution of carbon into austenite during austenitization, a substantially fully pearlitic as-cast microstructure is usually used as a precursor in the production of ADI in order to obtain short diffusion distances from high-carbon (cementite Fe3C) to low-carbon (ferrite) areas. In a ferritic ductile iron, carbon has to diffuse several orders of magnitude further from the graphite nodules into the austenite matrix which is formed, requiring a considerably longer austenitizing time and/or a higher austenitizing temperature.
ADI castings are, compared to conventional ductile iron, at least twice as strong at the same ductility level, or show at least twice the ductility at the same strength level. Compared to steel castings of the same strength, the cost of casting and heat treatment for ADI is much lower, and simultaneously the machinability is improved, especially if conducted before heat treatment. High-strength ADI cast alloys are therefore increasingly being used as a cost-efficient alternative to welded structures or steel castings, especially since components made from steel are heavier and more expensive to manufacture and finish than components made from ADI.
The superior mechanical properties of ADI emanate from an ausferritic microstructure of very fine needles of acicular ferrite in a matrix of austenite, thermodynamically stabilized by a high carbon content. The much higher silicon content in austempered ductile irons, compared to common steels, stabilizes carbon in graphite instead of cementite (Fe3C), thus preventing the precipitation of carbides as long as the austempering is not too prolonged.
The chemical composition of ADI is similar to that of conventional ductile iron, i.e. about 3.4-3.8 weight-% carbon, 2.3-2.7 weight-% silicon, 0.3-0.4 weight-% manganese, a maximum of 0.015 weight-% sulphur and a maximum of 0.06 weight-% phosphorus. Depending on casting thickness, alloying elements such as up to 0.8 weight-% copper, up to 2.0 weight-% nickel, and up to 0.3 weight-% molybdenum are usually added to the base composition, to avoid the detrimental formation of pearlite due to less rapid cooling rate in the core from the austenitization temperature to the austempering temperature.
The Standard Specification for Austempered Ductile Iron Castings (Designation: A 897M-06, published by the American Society for Testing and Materials (ASTM) on Apr. 3, 2006) states (on page 7, table X1.2) that silicon is one of the most important elements in ADI because it promotes graphite formation, decreases the solubility of carbon in austenite, increases the eutectoid temperature and inhibits the formation of bainitic carbide. It also warns that excessively high levels of silicon can suppress ausferrite in localized areas by stabilizing ferrite. The recommended amount of silicon is therefore given as 2.50%±0.20%. The heat treatment parameters (temperatures, hold times and cooling rates) are not specified in the standards, but in the scientific literature the emphasis has been on the effects of austempering, while the preceding austenitizing step has caught less attention. For conventional ADI with Si levels less than 3.35% the austenitizing temperature has usually not exceeded 900° C., and in the few attempts that have been made to austemper ductile irons with high silicon amounts between 3.35% Si and 4.60% Si, the austenitizing temperatures have been below 910° C.